Я пробовал так -
(require 'auto-complete-config)
auto-complete-config
(global-auto-complete-mode t)
t
(ac-config-default)
t
(require 'ac-slime)
ac-slime
(slime-mode t)
t
(add-hook 'slime-mode-hook 'set-up-slime-ac)
(set-up-slime-ac slime-autodoc-maybe-enable slime-add-easy-menu slime-setup-first-change-hook)
Но в mode line в lisp-буферах и slime-буфере AC нет